Klystrons

ABSTRACT

A klystron having a collector (2) and a drift tube (1) is provided with a vacuum retaining ceramic tube (8) between the drift tube and the collector. The outer surface of the cermaic tube is coated with metal M and spaced from the inside surface of the ceramic tube is a deformable metal cylinder (16). A transmission line TL is created and the cylinder (16) is so bent that the transmission line is a half wave length long at the midfrequency of the operating bandwidth.

United States Patent [191 Edgcombe [4 Dec. 3, 1974 1 KLYSTRONS [75] I Inventor: Christopher John Edgcombe,

Cambridge, England [73] Assignee: English Electric Valve Company Limited, Essex, England 221 Filed: Oct. 11, 1973 2 1 App]. No.: 405,545

[30] Foreign Application Priority Data Oct. 11, 1972 Great Britain 4675/72 [52] US. Cl 315/554, 315/538, 315/3953 [51] Int. Cl. H0lj 23/20 [58] Field of Search 315/538, 5.39, 3.54, 39.53

[56] References Cited UNITED STATES PATENTS 3,392,301 7/1968 Campbell 315/538 3,392,303 7/1968 Wolff et a1 315/539 10 an 7 L 3,436,586 4/1969 Gross 315/3.5 3,448,331 6/1969 Cook 315/3953 3,500,111 3/1970 OLonghlin et al.... 315/538 3,753,031 Hervier et a1 315/3.5

Primary Examiner-Archie Borchelt Assistant Examiner-Saxfild Chatamen, Jr. Attorney, Agent, or Firm-Baldwin, Wight & Brown 1 5 7 ABSTRACT A klystron having a collector (2 and a drift tube (1) is provided with a vacuum retaining ceramic tube (8) between the drift tube and the collector. The outer surface of the cermaic tube is coated with metal M and spaced from the inside surface of the ceramic tube is a deformable metal cylinder (16). A transmission line TL is created and the cylinder (16) is so bent that the transmission line is a half wave length long at the mid-frequency of the operating bandwidth.

7 Claims, 3 Drawing Figures Pmtmmm 3M4 3,852.636

sum 10F 2 FIG. I.

FIG. 2

' PATENIELBEB 131914 1852.636

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KLYSTRONS This invention relates to Klystrons of the so-called metal-insulation envelope type, i.e., of the type wherein the electron beam, after passing the frift tubes of the klystron, enters a hollow collector which is spaced from the adjacent end of the last drift tube by a member made of insulating material in current practice of ceramic material which forms parts of the vacuumretaining envelope of the klystron.

In a klystron of this type high frequency radiation will, when the tube is in operation, escape through the gap between the last drift tube and the collector and thus make the tube a source of interference with nearby other apparatus unless means are provided for preventing such'radiation occurring. FIG. 1 of the accompanying drawings is a schematic representation, half in section in the centre line, showing, so far as is necessary to an understanding thereof, a known construction of klystron having such means. This figure shows the adjacent ends of the last drift tube and of the collector together with the parts adjacent the gap between said drift tube and collector. This known construction is not only economical, convenient to manufacture and neat in appearance but, as will be more clearly seen later, has the important practical advantages that it does not involve any need for small clearances between parts which are inside the evacuated envelope space, does not require the adoption of close manufacturing'tolerances, and is not liable to the occurrence' of "short circuits as a result of thermal expansion or distortion of parts.

FIG. 1 illustrates the last drift tube at the collector the collector of the last drift tube of the klystron is referenced l,- and 2 is the hollow collector. The end of said collector nearer said drift tube is tapered inwardly in the customary way. 3 is a cap on the electron entry end of'the collector for preventing secondary electrons from the interior thereof travelling back to the drift tube. It defines the entry aperture into the collector which, in the construction shown, is water jacketed for cooling. The water jacketspace is indicated by the reference letter W. It is the space between the collector 2 and the parts referenced 4 and 5 and it contains a partition wall 6 so that water circulation is up one side of this wall anddown the other. Because the water jacket extends practically to the end of the collector, very effective water cooling is obtained. A water jacket space 'W' is also provided round the drift tube 1. g The drifttubel is seated in a mounting flange member 7 made of ferro-magnetic material and forming part of the usual focussing magnetic circuit of the klystron. The flange '7 is parallel to and spaced from the parts 3 and 4. A' ceramic cylinder 8 is provided between the flange 7 and a flange 9 extending outwardly from the outer, wall of the collector water jacket. The said cylinder 8 has metal coatings M on both its inner and outer surfaces. These coatings may be obtained for example by metallisation by firing on molybdenum and then nickel plating it. The inner coating stops short of the end of cylinder 8 adjacent the flange 9 and the outer coating stops short of the end of said cylinder adjacent the flange 7. The said cylinder 8 is fixed in position by edge of the ring member to the inside as shown. Over. the tape is placed a wrapping of aluminum or other suitable metal foil which is insulated from the ring member 10 by the tape but is in good electrical contact with the metal coating on the outer face of the ceramic cylinder 8. An encircling tensioned neoprene rubber or similar elastic band 14 ensures thev maintenance of good electrical contact. A similar but wider elastic band 15, of such width as to extend over and between the ring members 10 and 11, holds the foil 13 tight against the tape 12 and provides an outer cover between said ring members 10 and 11 The inner coating M on the ceramic cylinder is in good electrical contact with the ring member 10 and the outer coating M is in good electrical contact with the ring member 11.

It will be seen that the effect of this known construction' is to provide a high frequency transmission line which extends, as indicated by the arrow headed broken line TL, radially outwards along the gap between the parts 1 and 7 on the one hand and the parts 3 and 4 on the other; then, at right angles to this portion of its length, down between the parts 5 and 8; and then turns through and extends up along the ceramic filled space between the inner and outer coatings M. This transmission line is short circuited as respects high frequency at its outer end, i .e., the end outside the vacuum, by the capacitor of which the tape 12 is the dielectric and the other ring member 10 and foilwrapping 13 are the electrodes. If the electric length of this transmission line isa half wave length long (or an integral plurality of half wavelengths long) at the intended working frequency f of the klystron then, at this frequency the high frequency short circuit provided by the aforesaid capacitor, although physically outside the vacuum and at the outer end of the transmission line, will appear as a high frequency short circuit at the inner end of the transmission line and will therefore in effect connect the collector directly to the adjacent drift tube as respects the working high frequency. Theoretically therefore, high frequency radiation through the gap should be prevented.

In practice, however, it is found that, although high frequency radiation at most frequencies in the intended working frequency band centred on the frequency f is prevented or, at any rate, greatly reduced, there is a liability for considerable radiation to occur at one or more spot frequencies inside the working band usually near the ends thereof especially if the band is j a wide one. Indeed, in some cases, the radiation at one or more spot frequencies has been found to be greater,-

when the above described radiation-preventing means are provided, than if they were not provided at all. The reasons for this will now be explained.

In the known construction of FIG. 1 the length of the transmission line is, as already stated, composed of three portions namely the radial length portion along the gap between the part 7 on the one hand and the parts 3 and 4 on the other; the perpendicular length portion between the parts 5 and 8; and the final length portion up through the dielectric-filled space between the inner and outer coatings M. Accordingly, the design criterion adopted has been merely to make the combined length of these three portions equal to one half wave length at the frequency f and the dimensions have been chosen to satisfy this criterion. However, there are other factors affecting the choice of the dimensions of the parts of a klystron, notably the requirement that the shape of the collector shall be properly chosen adequately to dissipate heat. Another factor affecting choice of dimensions is that of intended operating voltage: in general two tubes of different operating voltages will be differently dimensioned even although they are of the same intended operating frequency. When a tube has been designed and made to dimensions chosen to suit the various factors involved, although the dimensions are such as to make the electrical length of the transmission line a half wave length at the frequency f, there will be in most cases, and in practically all cases in which the operating frequency band is a wide one, or or more spot frequencies within that band at which the electrical length of the transmission line approximates to a quarter wave length, or an integral odd number of quarter wavelengths at the spot frequency in question or at the second harmonic thereof, i.e., at twice said spot frequency. For such a spot frequency the short circuit provided by the capacitor outside the vacuum will appear as, or approximate to, a high frequency open circuit at the inner end of the transmission line and objectionable radiation at the spot frequency, or the second harmonic thereof, will occur.

According to this invention a klystron of the metalinsulation type having a hollow collector which is spaced from the adjacent end of the last drift tube by an insulating member which forms part of the evacuated envelope is provided withmeans for preventing high frequency radiation from the gap between the last drift tube and the collector, said means being constituted by a high frequency transmission line which is external with respect to'said gap, is short-circuited (as respects high frequency) at its outer end outside the evacuated space of the klystron, is electrically an integral number (including unity) of half wave lengths long at the centre frequency (f) of the intended working frequency band of the klystron and is also so dimensioned that it does not correspond to an electrical length which approximates to an odd number (including unity) of quarter wave lengths of any frequency or second harmonic of any frequency inside the said working band.

Preferably the parts constituting the transmission line include an adjustable metal member which does not form part of the evacuated envelope of the klystron and which can be adjusted (during manufacture) to modify the electrical constants of the line. Embodiments of this nature have the practical advantage that, after the parts of the klystron have been made and assembled (but before evacuation) a high frequency source providing a frequency variable over a band corresponding with the intended working frequency band may be applied to the assembled klystron, the occurrence of radiation from the gap observed, and the adjustable member ad- 5 justed until radiation within band is bandis eliminated .or reduced to an acceptably low level, there being no need to modify or change the dimensions of any part of the envelope of the klystron in order to achieve this result.

The adjustable member is preferably a deformable metal member inside the insulating member of the envelope and spaced therefrom, so that the aforesaid final length of the line has dielectric in part consisting of solid dielectric (the material of the insulating member of the envelope) and in part consisting of vacuum (the space between the insulating member and the deformable member which is spaced from the inner wall of said insulating member). Although it is possible to make said spaced adjustable metal member of telescopic or of bellows-like construction so that it may be adjusted by changing its physical length i.e., its length parallel to the klystron axis, it is very difficult to make such a member able to retain the length to which it,has been adjusted without resorting to the added complication of providing adjustable means for positively holding the said member at the length to which it has been adjusted, and it is considered that such added complication would not justify the expense involved in its provision. It is preferred, therefore, to constitute the spaced adjustable member by a simple cylinder of deformable metal and to deform it as required by bending it to modify its spacing from the inner wall of the insulating member of the envelope.

In one embodiment of the invention the insulating member of the envelope is provided with a metal coating which is on its outer surface only and is connected to a metal part of the klystron structure on one side only of the gap and a deformable metal cylinder connected to a metal part of the klystron structure on the other side only of said gap is provided inside said insulating member and spaced from the inner wall thereof, the length of the transmission line being thus made up of the sum of the radial length of the gap, the length of a space communicating with said gap and situated between a part of the outer surface of the collector structure and said deformable metal cylinder, and the length, parallel to the axis of the klystron, of the separation between said cylinder and the metal coating on the outer surface of the insulating member.

In another embodiment of the invention there is no deformable member and the insulating member is provided with a metal coating which is on its outer surface only and is connected to a metal part of the klystron structure on one side only of the gap, the inner surface being either free of any metal coating or, preferably, being provided with a metal coating extending only over that portion of said inner surface which is in the region of the gap, said coating being connected to a metal part of the klystron on the side of the gap other than that to which the outer coating is connected. The reason why it is preferred to provide a short inner metal coating in the stated region (rather than omitting the inner coating altogether) is that, in operation of the klystron, metal deposition is apt to occur over the region in question and if this region is already coated such metal deposition will not change the electrical constants of the transmission line. Although embodiready described.

ments without an adjustable member do not have the advantage of the possibility of adjustment during manufacture, they are nevertheless quite satisfactory if dimensioned to satisfy the requirements of this invention as respects transmission line length.

The invention also provides a method of adjusting a klystron in accordance with the invention and having an adjustable member for adjusting the electrical constants of the transmission line. This method consists of inserting a probe axially in the assembled klystron (before itis evacuated), supplying high frequency energy including frequencies within the intended operating frequency band of the klystron from a high frequency source to said probe, observing the frequency or frequencies at which radiation occurs from the gap, and adjusting the adjustable member until there is no radiation exceeding a predetermined acceptable level at any frequency within the band. Observation of radiation may be effected by a radiation detector outside the klystron or, more simply, by observing the loading on the high frequency source for, obviously, this will be increased when radiation occurs.

If the klystron is water-cooled the aforesaid part of the outersurface of the collector structure may conveniently be part of the outer wall of the cooling water jacket of the collector.

Preferably the high frequency short. circuit at the outer end of the transmission line is provided by a capacitor including a foil wrapping which is in electrical contact with metal coating on the outer surface of the insulating member of the envelope and extends over In FIG. 3 the inner metal coating M, shown in FIG. 1 on the inner surface of the insulating member 8, is omitted except for the small area, referenced M in F IG'. 3, in the region of the gap. As will be apparent, the length of the transmission line in FIG. 3 is constituted by the radical length of the gap plus the electrical length of the part of the path in the ceramic. The parts of the construction in FIG. 3, which is not adjustable, are, of course, dimensioned to satisfy the requirements of the invention as already set forth.

I claim:

1. In a klystron structure of the type including a high 7 frequency interaction section terminating in a last drift FIGS. 2 and 3 of the accompanying drawings. Like references denote like parts in all three figures. Although FIGS. 2 and 3 show water-cooled klystrons the invention is, of course, equally applicable to klystrons which are not water-cooled.

In view of the detailed description already given of the known construction shown in FIG. 1, the embodiments' of the invention shown in FIGS. 2 and 3 can be -most-easily described by pointing out the respects in which they differ from the construction of FIG. 1.

In FIG. 2 the inner metal coating M which, in FIG. 1, is provided on, the ceramic member 8, is omitted and there is provided a deformable metal cylinder 16 inside and spaced from the inner wall of the said member 8.

This cylinder, which is such that it can be deformed by I so bent that the transmission line is a half wave length long at the mid-frequency f of the intended working frequency band and is not and does not approximate to an odd number (including unity) of quarter wave lengths long at any frequency within said band or at the secondharmonic of any frequency within said band. The bending of the cylinder 16 is preferably effected with the aid of a high frequency source and probe as altube, said interaction section operating over a selected frequency band, a collector section spaced from said last drift tube to define a gap through which high frequency radiation within said frequency band tends to escape, a sleeve-like insulating member surrounding an end of said collector section and interposed between said sections to preserve evacuated condition within saidfklystron structure, a transmission line having at least a first section defined between said last drift tube and said collector section and leading radially outwardly of said gap and an end section extending parallel to the axis of the klystron structure, and high frequency short circuit means connectedto said end section for providing a high frequency short circuit located externally of said klystron structure, the improvement wherein: said end section of the transmission line is defined by a metallic film in surrounding contiguous relation to said insulating member and a tubular metallic surface within the evacuated interior of said klystron structure, and said metallic surface being spaced to provide a dielectric within and along the length of said end section of said transmission line which is constituted partially by said insulating member and partially by the vacuum within the klystron structure, said end section being so dimensioned that said transmission line is'of electrical length which is an integral number (including unity) of half wavelengths of the center frequency t of said frequency band and also does not correspond to an electrical length approximately to an odd number (including unity) of quarter wavelengths of any frequency or of the second harmonic of any frequency within said frequency band.

2. A klystron as claimed in claim 1, wherein said metallicfilm is constituted by a metal coating over the major part of the length of the external surface of said insulating member and said metallic surface is provided by a metal member inside and spaced from the inner surface of said insulatingmember in surrounding relation to and spaced from said endof the collector section so as to provide an intermediate sectionof said transmission line between said metal member and said end of the collector section which joins said first and end sections, which metal member does not form part of the evacuated envelope of the klystron, and is adjustable to modify the electrical constants of the transmission line. I

3. A klystron as claimed in claim 2, wherein the metal member is collapsible so that it may be adjusted by changing its physical length.

4. A klystron as claimed in claim 2, wherein the metal member is a deformable cylinder which can be bent to modify its spacing from the inner wall of the insulating member.

5. A klystron according to claim 1, wherein said metallic film is constituted by a metal coating extending over at least the major part of the outer surface of the insulating member and connected to a metal part of the klystron structure on one side of the gap and wherein the inner surface of said insulating member is provided with a metal coating extending only over that portion of said inner surface which is in the region of the gap and connected to a metal part of the klystron on the side of the gap other than that to which the coating first mentioned is connected, and said metallic surface being provided by the outer surface of said end of the collector section whereby said first section of the transmission line leads directly into said end section thereof.

6. A klystron as claimed in claim 1, wherein the high frequency short-circuit at the outer end of the transmission line is provided by a capacitor including a foil wrapping which is in electrical contact with said metallic film and extends over but is insulated, as respects DC, from a metal member in electrical connection with the last drift tube 'of the klystron.

l. The method of making a klystron which comprises the steps of;

a. assembling component parts of the klystron preparatory to evacuating the envelope thereof, the components including a collector section having a last drift tube defining a gap with said collector section, an insulating sleeve separating said sections and surrounding said gap, and a metallic sleeve member between said insulating sleeve and said collector section;

b. inserting a probe axially into the assembled klystron and supplying high frequency energy including frequencies within the intended operating frequency band of the klystron to said probe;

c. observing the frequencies at which radiation occurs from the gap;

(1. adjusting said metallic sleeve member until there is no radiation exceeding a predetermined acceptable level at any frequency within said band; and thereafter e. evacuating and sealing said klystron.

PATENT @EFFKQE @ERTEFKQATE @F @QREQ'HN Patent No. I 3 ,852, 636 Dated December 3, 1974 Inventor(s) Christopher John Edgcombe It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

[30} Foreign Application Priority Data Change Britain 4675/72 to Britain 46751/72 sighed and sealed this 38th day of February 1975.

(SEAL) Attest 2 C. MARSHALL DANN RUTH C. MASGN Commissioner of Patents Attesting Officer and Trademarks I FORM P 0 0 (10-69) USCOMM-DC 60376-P89 UIS. GOVERNMENT PRINTING OFFICE: I569 O-36$-334. 

1. In a klystron structure of the type including a high frequency interaction section terminating in a last drift tube, said interaction section operating over a selected frequency band, a collector section spaced from said last drift tube to define a gap through which high frequency radiation within said frequency band tends to escape, a sleeve-like insulating member surrounding an end of said collector section and interposed between said sections to preserve evacuated condition within said klystron structure, a transmission line having at least a first section defined between said last drift tube and said collector section and leading radially outwardly of said gap and an end section extending parallel to the axis of the klystron structure, and high frequency short circuit means Connected to said end section for providing a high frequency short circuit located externally of said klystron structure, the improvement wherein: said end section of the transmission line is defined by a metallic film in surrounding contiguous relation to said insulating member and a tubular metallic surface within the evacuated interior of said klystron structure, and said metallic surface being spaced to provide a dielectric within and along the length of said end section of said transmission line which is constituted partially by said insulating member and partially by the vacuum within the klystron structure, said end section being so dimensioned that said transmission line is of electrical length which is an integral number (including unity) of half wavelengths of the center frequency of said frequency band and also does not correspond to an electrical length approximately to an odd number (including unity) of quarter wavelengths of any frequency or of the second harmonic of any frequency within said frequency band.
 2. A klystron as claimed in claim 1, wherein said metallic film is constituted by a metal coating over the major part of the length of the external surface of said insulating member and said metallic surface is provided by a metal member inside and spaced from the inner surface of said insulating member in surrounding relation to and spaced from said end of the collector section so as to provide an intermediate section of said transmission line between said metal member and said end of the collector section which joins said first and end sections, which metal member does not form part of the evacuated envelope of the klystron, and is adjustable to modify the electrical constants of the transmission line.
 3. A klystron as claimed in claim 2, wherein the metal member is collapsible so that it may be adjusted by changing its physical length.
 4. A klystron as claimed in claim 2, wherein the metal member is a deformable cylinder which can be bent to modify its spacing from the inner wall of the insulating member.
 5. A klystron according to claim 1, wherein said metallic film is constituted by a metal coating extending over at least the major part of the outer surface of the insulating member and connected to a metal part of the klystron structure on one side of the gap and wherein the inner surface of said insulating member is provided with a metal coating extending only over that portion of said inner surface which is in the region of the gap and connected to a metal part of the klystron on the side of the gap other than that to which the coating first mentioned is connected, and said metallic surface being provided by the outer surface of said end of the collector section whereby said first section of the transmission line leads directly into said end section thereof.
 6. A klystron as claimed in claim 1, wherein the high frequency short-circuit at the outer end of the transmission line is provided by a capacitor including a foil wrapping which is in electrical contact with said metallic film and extends over but is insulated, as respects DC, from a metal member in electrical connection with the last drift tube of the klystron.
 7. The method of making a klystron which comprises the steps of; a. assembling component parts of the klystron preparatory to evacuating the envelope thereof, the components including a collector section having a last drift tube defining a gap with said collector section, an insulating sleeve separating said sections and surrounding said gap, and a metallic sleeve member between said insulating sleeve and said collector section; b. inserting a probe axially into the assembled klystron and supplying high frequency energy including frequencies within the intended operating frequency band of the klystron to said probe; c. observing the frequencies at which radiation occurs from the gap; d. adjusting said metallic sleeve member until there is no radiation exceeding a predetermIned acceptable level at any frequency within said band; and thereafter, e. evacuating and sealing said klystron. 